Runt-related transcription factor 3, or RUNX3, is a tumor suppressor in gastric cancer. Inactivation of RUNX3 is causally associated with the genesis of gastric cancer, since RUNX3 is frequently inactivated in gastric cancers. Infection with Helicobacter pylori is the strongest risk factor for the development of gastric cancer. Recent studies have indicated that H. pylori infection plays an important role in the inactivation of RUNX3, and that this inactivation contributes to the pathogenesis of H. pylori. We recently demonstrated that H. pylori inactivate RUNX3 by inducing its ubiquitination and degradation during an early stage of the infection in a virulence factor CagA-dependent manner. However, the detailed molecular mechanism and the physiological function of this inactivation in the development of gastric cancer remain unclear. The overall goal of this proposal is to investigate how H. pylori utilize CagA to inactivate RUNX3 and the contribution of this inactivation to the formation of gastric cancer. In Specific Aim 1, we will determine how H. pylori CagA induce the ubiquitination and degradation of RUNX3. We will investigate whether nuclear export of RUNX3 is a prerequisite for its degradation and identify the CagA-associated ubiquitin E3 ligase for RUNX3. In Specific Aim 2, we will determine how H. pylori CagA interact with RUNX3 and whether blocking the interaction could prevent the formation of H. pylori-induced gastric cancer. These cell-based and in vivo animal experiments will provide new insights into the role of H. pylori CagA in the pathogenesis of H. pylori and give a better understanding of the role RUNX3 plays as a tumor suppressor in gastric cancer.